While it's certainly possible to have a throttle most electric bikes require pedaling and therefore some degree of exercise. In many cases it gets people riding that would not be riding otherwise. For people that are actually interested in fitness it's a lot easier to maintain zone 2 on real roads with electric assist without the impulse to push it a bit up the hills. I did far more aggressive rides when I had access to my electric road bike than I do on other bikes because I knew I had a bail out if I pushed too far. Nobody's going to congratulate an e-bike rider on their feats of endurance or power but there are lots of points between a normal cycling effort and no effort at all.